Machine-Level Programming V:Miscellaneous Topics

Machine-Level Programming V:Miscellaneous Topics

Topics Buffer Overflow Linux Memory Layout Understanding Pointers Floating-Point Code

CS 105Tour of Black Holes of Computing

– 2 – 105

Internet Worm and IM WarInternet Worm and IM WarNovember, 1988

Internet Worm attacks thousands of Internet hosts. How did it happen?

July, 1999 Microsoft launches MSN Messenger (instant messaging

system). Messenger clients can access popular AOL Instant

Messaging Service (AIM) servers

AIMserver

AIMclient

AIMclient

MSNclient

MSNserver

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Internet Worm and IM War (cont.)Internet Worm and IM War (cont.)August 1999

Mysteriously, Messenger clients can no longer access AIM servers.

Microsoft and AOL begin the IM war:AOL changes server to disallow Messenger clientsMicrosoft makes changes to clients to defeat AOL changes.At least 13 such skirmishes.

How did it happen?

The Internet Worm and AOL/Microsoft War were both based on stack buffer overflow exploits!

Many Unix functions do not check argument sizes.Allows target buffers to overflow.

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String Library CodeString Library Code Implementation of Unix function gets

No way to specify limit on number of characters to read

Similar problems with other Unix functionsstrcpy: Copies string of arbitrary lengthscanf, fscanf, sscanf, when given %s conversion specification

/* Get string from stdin */char *gets(char *dest){ int c = getc(); char *p = dest; while (c != EOF && c != '\n') { *p++ = c; c = getc(); } *p = '\0'; return dest;}

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Vulnerable Buffer CodeVulnerable Buffer Code

int main(){ printf("Type a string: "); echo(); return 0;}

/* Echo Line */void echo(){ char buf[4]; /* Way too small! */ gets(buf); puts(buf);}

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Buffer Overflow ExecutionsBuffer Overflow Executions

unix>./bufdemoType a string:123123

unix>./bufdemoType a string:12345Segmentation Fault

unix>./bufdemoType a string:12345678Segmentation Fault

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Buffer Overflow StackBuffer Overflow Stack

echo:pushl %ebp # Save %ebp on stackmovl %esp,%ebpsubl $20,%esp # Allocate space on stackpushl %ebx # Save %ebxaddl $-12,%esp # Allocate space on stackleal -4(%ebp),%ebx # Compute buf as %ebp-4pushl %ebx # Push buf on stackcall gets # Call gets. . .

/* Echo Line */void echo(){ char buf[4]; /* Way too small! */ gets(buf); puts(buf);}

Return Address

Saved %ebp

[3][2][1][0] buf

%ebp

StackFrame

for main

StackFrame

for echo

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Buffer Overflow Stack ExampleBuffer Overflow Stack Example

Before call to gets

unix> gdb bufdemo(gdb) break echoBreakpoint 1 at 0x8048583(gdb) runBreakpoint 1, 0x8048583 in echo ()(gdb) print /x *(unsigned *)$ebp$1 = 0xfffff8f8(gdb) print /x *((unsigned *)$ebp + 1)$3 = 0x804864d

8048648: call 804857c <echo> 804864d: mov 0xffffffe8(%ebp),%ebx # Return Point

Return Address

Saved %ebp

[3][2][1][0] buf

%ebp

StackFrame

for main

StackFrame

for echo

0xfffff8d8

Return Address

Saved %ebp

[3][2][1][0] buf

StackFrame

for main

StackFrame

for echo

ff ff f8 f8

08 04 86 4d

xx xx xx xx

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Buffer Overflow Example #1Buffer Overflow Example #1

Before Call to gets Input = “123”

No Problem

0xfffff8d8

Return Address

Saved %ebp

[3][2][1][0] buf

StackFrame

for main

StackFrame

for echo

ff ff f8 f8

08 04 86 4d

00 33 32 31

Return Address

Saved %ebp

[3][2][1][0] buf

%ebp

StackFrame

for main

StackFrame

for echo

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Buffer Overflow Stack Example #2Buffer Overflow Stack Example #2

Input = “12345”

8048592: push %ebx 8048593: call 80483e4 <_init+0x50> # gets 8048598: mov 0xffffffe8(%ebp),%ebx 804859b: mov %ebp,%esp 804859d: pop %ebp # %ebp gets set to invalid value 804859e: ret

echo code:

0xfffff8d8

Return Address

Saved %ebp

[3][2][1][0] buf

StackFrame

for main

StackFrame

for echo

ff ff 00 35

08 04 86 4d

34 33 32 31

Return Address

Saved %ebp

[3][2][1][0] buf

%ebp

StackFrame

for main

StackFrame

for echo

Saved value of %ebp set to 0xffff0035

Bad news when later attempt to restore %ebp

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Buffer Overflow Stack Example #3Buffer Overflow Stack Example #3

Input = “12345678”

Return Address

Saved %ebp

[3][2][1][0] buf

%ebp

StackFrame

for main

StackFrame

for echo

8048648: call 804857c <echo> 804864d: mov 0xffffffe8(%ebp),%ebx # Return Point

0xfffff8d8

Return Address

Saved %ebp

[3][2][1][0] buf

StackFrame

for main

StackFrame

for echo

38 37 36 35

08 04 86 00

34 33 32 31

Invalid address

No longer pointing to desired return point

%ebp and return address corrupted

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Malicious Use of Buffer OverflowMalicious Use of Buffer Overflow

Input string contains byte representation of executable code Overwrite return address with address of buffer When bar() executes ret, will jump to exploit code

void bar() { char buf[64]; gets(buf); ... }

void foo(){ bar(); ...}

Stack after call to gets()

B

returnaddress

A

foo stack frame

bar stack frame

B

exploitcode

pad

data written

bygets()

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Exploits Based on Buffer OverflowsExploits Based on Buffer OverflowsBuffer overflow bugs allow remote machines to execute

arbitrary code on victim machines.

Internet worm Early versions of the finger server (fingerd) used gets() to

read the argument sent by the client:finger geoff@cs.hmc.edu

Worm attacked fingerd server by sending phony argument:finger “exploit-code padding new-return-address”exploit code: executed a root shell on the victim machine with a

direct TCP connection to the attacker.

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Exploits Based on Buffer OverflowsExploits Based on Buffer OverflowsBuffer overflow bugs allow remote machines to execute

arbitrary code on victim machines.

IM War AOL exploited existing buffer overflow bug in AIM clients Exploit code: returned 4-byte signature (the bytes at some

location in the AIM client) to server. When Microsoft changed code to match signature, AOL

changed signature location.

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Avoiding Overflow VulnerabilityAvoiding Overflow Vulnerability

Use library routines that limit string lengths fgets instead of gets strncpy instead of strcpy

Or strlcpy if available (see man strcpy and use –lbsd) Don’t use scanf with %s conversion specification

Use fgets to read the string

/* Echo Line */void echo(){ char buf[4]; /* Way too small! */ fgets(buf, sizeof buf, stdin); fputs(buf, stdout);}

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Linux Memory LayoutLinux Memory LayoutStack

Runtime stack

Heap Dynamically allocated storage When call malloc, calloc, realloc, new

DLLs Dynamically Linked (Shared) Libraries Library routines (e.g., printf, malloc) Linked into object code when first executed

Data Statically allocated data E.g., arrays & strings declared in code

Text Executable machine instructions Read-only

Upper 2 hex digits of address

FF

BF

7F

3F

C0

80

40

00

(Stack)

DLLs

TextData

Heap

Heap

08

Stack

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Linux Memory AllocationLinux Memory Allocation

LinkedFF

7F

3F

80

40

00

Stack

DLLs

TextData

08

Some Heap

FF

7F

3F

80

40

00

Stack

DLLs

TextData

Heap

08

MoreHeap

FF

7F

3F

80

40

00

Stack

DLLs

TextData

Heap

Heap

08

InitiallyFF

7F

3F

80

40

00

Stack

TextData

08

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Text & Stack ExampleText & Stack Example

(gdb) break main(gdb) run Breakpoint 1, 0x804856f in main ()(gdb) print $esp $3 = (void *) 0xfffffc78

Main Address 0x804856f should be read 0x0804856f

Stack Address 0xfffffc78

InitiallyFF

7F

3F

80

40

00

Stack

TextData

08

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Dynamic Linking ExampleDynamic Linking Example(gdb) print malloc $1 = {<text variable, no debug info>} 0x8048454 <malloc>(gdb) run Program exited normally.(gdb) print malloc $2 = {void *(unsigned int)} 0x40006240 <malloc>

Initially Code in text segment that invokes dynamic

linker Address 0x8048454 should be read 0x08048454

Final Code in DLL region

LinkedFF

7F

3F

80

40

00

Stack

DLLs

TextData

08

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Memory Allocation ExampleMemory Allocation Example

char big_array[1<<24]; /* 16 MB */char huge_array[1<<28]; /* 256 MB */

int beyond;char *p1, *p2, *p3, *p4;

int useless() { return 0; }

int main(){ p1 = malloc(1 << 28); /* 256 MB */ p2 = malloc(1 << 8); /* 256 B */ p3 = malloc(1 << 28); /* 256 MB */ p4 = malloc(1 << 8); /* 256 B */ /* Some print statements ... */}

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Example AddressesExample Addresses

$esp 0xfffffc78p3 0x500b5008p1 0x400b4008Final malloc 0x40006240p4 0x1904a640 p2 0x1904a538beyond 0x1904a524big_array 0x1804a520huge_array 0x0804a510main() 0x0804856fuseless() 0x08048560Initial malloc 0x08048454

FF

7F

3F

80

40

00

Stack

DLLs

TextData

Heap

Heap

08

– 22 – 105

C OperatorsC OperatorsOperators Associativity() [] -> . left to right! ~ ++ -- + - * & (type) sizeof right to left* / % left to right+ - left to right<< >> left to right< <= > >= left to right== != left to right& left to right^ left to right| left to right&& left to right|| left to right?: right to left= += -= *= /= %= &= ^= != <<= >>= right to left, left to right

Note: Unary +, -, and * have higher precedence than binary forms

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C Pointer DeclarationsC Pointer Declarationsint *p p is a pointer to int

int *p[13] p is an array[13] of pointer to int

int *(p[13]) p is an array[13] of pointer to int

int **p p is a pointer to a pointer to an int

int (*p)[13] p is a pointer to an array[13] of int

int *f() f is a function returning a pointer to int

int (*f)() f is a pointer to a function returning int

int (*(*f())[13])() f is a function returning ptr to an array[13] of pointers to functions returning int

int (*(*x[3])())[5] x is an array[3] of pointers to functions returning pointers to array[5] of ints

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Data Representations: IA32 + x86-64Data Representations: IA32 + x86-64Sizes of C Objects (in Bytes)

C Data Type Typical 32-bit Intel IA32 x86-64unsigned int 4 4

4 int 4 4

4 long int 4 4

8char 1 1

1short 2 2

2 long long 8 8

8 float 4 4

4double 8 8

8 long double 8 10/12

16char * 4 4

8Or any other pointer

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%rax

%rbx

%rcx

%rdx

%rsi

%rdi

%rsp

%rbp

x86-64 Integer Registersx86-64 Integer Registers

Extend existing registers. Add 8 new ones. Make %ebp/%rbp general purpose

%eax

%ebx

%ecx

%edx

%esi

%edi

%esp

%ebp

%r8

%r9

%r10

%r11

%r12

%r13

%r14

%r15

%r8d

%r9d

%r10d

%r11d

%r12d

%r13d

%r14d

%r15d

%ax

%bx

%cx

%dx

%si

%di

%sp

%bp

%r8w

%r9w

%r10w

%r11w

%r12w

%r13w

%r14w

%r15w

%dx

%si

%di

%sp

%bp

%r8w

%r9w

%r10w

%r11w

%r12w

ah al

bh bl

ch cl

dh dl

sil

dil

spl

bpl

r8b

r9b

r10b

r11b

r12b

r13b

r14b

r15b

Oh.

My.

God.

– 26 – 105

%rax

%rbx

%rcx

%rdx

%rsi

%rdi

%rsp

%rbp

x86-64 Integer Registersx86-64 Integer Registers

Extend existing registers. Add 8 new ones. Make %ebp/%rbp general purpose

%eax

%ebx

%ecx

%edx

%esi

%edi

%esp

%ebp

%r8

%r9

%r10

%r11

%r12

%r13

%r14

%r15

%r8d

%r9d

%r10d

%r11d

%r12d

%r13d

%r14d

%r15d

– 27 – 105

InstructionsInstructions

Long word l (4 Bytes) ↔ Quad word q (8 Bytes)

New instructions: movl → movq addl → addq sall → salq movzbq, movslq etc.

32-bit instructions that generate 32-bit results: Set higher order bits of destination register to 0 Example: addl, movl (thus no movzlq)

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Swap in 32-bit ModeSwap in 32-bit Mode

void swap(int *xp, int *yp) { int t0 = *xp; int t1 = *yp; *xp = t1; *yp = t0;}

swap:pushl %ebpmovl %esp,%ebppushl %ebx

movl 12(%ebp),%ecxmovl 8(%ebp),%edxmovl (%ecx),%eaxmovl (%edx),%ebxmovl %eax,(%edx)movl %ebx,(%ecx)

movl -4(%ebp),%ebxmovl %ebp,%esppopl %ebpret

Body

Setup

Finish

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Swap in 64-bit ModeSwap in 64-bit Mode

Operands passed in registers (why useful?) First (xp) in %rdi, second (yp) in %rsi 64-bit pointers

No stack operations required

32-bit data Data held in registers %eax and %edx movl operation

void swap(int *xp, int *yp) { int t0 = *xp; int t1 = *yp; *xp = t1; *yp = t0;}

swap:movl (%rdi), %edxmovl (%rsi), %eaxmovl %eax, (%rdi)movl %edx, (%rsi)retq

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Swap Long Ints in 64-bit ModeSwap Long Ints in 64-bit Mode

64-bit data Data held in registers %rax and %rdx movq operation Otherwise same

void swap_l (long int *xp, long int *yp) { long int t0 = *xp; long int t1 = *yp; *xp = t1; *yp = t0;}

swap_l:movq (%rdi), %rdxmovq (%rsi), %raxmovq %rax, (%rdi)movq %rdx, (%rsi)retq

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New Calling ConventionsNew Calling Conventions

Most procedures no longer need stack frame

First six arguments passed in registers

Register %rbp available for general use

Stack frame accessed via %rsp

128 bytes below %rsp usable by function (“red zone”)

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IA32 Floating PointIA32 Floating Point

History 8086: first computer to implement IEEE FP

separate 8087 FPU (floating point unit) 486: merged FPU and Integer Unit onto one

chip

Summary Hardware to add, multiply, and divide Floating point data registers Various control & status registers

Floating Point Formats Single precision (C float): 32 bits Double precision (C double): 64 bits Extended precision (C long double): 80 bits

Instructiondecoder andsequencer

FPUInteger

Unit

Memory

– 33 – 105

FPU Data Register StackFPU Data Register Stack

FPU register format (extended precision)

s exp frac

063647879

FPU registers 8 registers Logically forms shallow

stack Top called %st(0) When push too many,

bottom values disappear

stack grows down

“Top” %st(0)

%st(1)

%st(2)

%st(3)

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FPU instructionsFPU instructions

Large number of floating point instructions and formats ~50 basic instruction types load, store, add, multiply sin, cos, tan, arctan, and log!

Sample instructions:

Instruction Effect Descriptionfldz push 0.0 Load zeroflds Addr push M[Addr] Load s.p. realfmuls Addr %st(0) <- %st(0)*M[Addr] Multiplyfaddp %st(1) <- %st(0)+%st(1); pop Add and pop

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Final ObservationsFinal Observations

Buffer Overflow Big C design mistake; most common security problem

Memory Layout OS/machine dependent (including kernel version) Stack/data/text/heap/DLL found in most machines

Type Declarations in C Notation obscure, but systematic

IA64 New registers, completely different calling conventions

IA32 Floating Point Strange “shallow stack” architecture